When boring is carried out with a rotary tool, chips remain inside the hole and the chips are heated and hardened by the cutting heat generated during the processing. Where the rotary tool comes into contact with the consolidated chips, a large cutting resistance acts upon the rotary tool and the rotary tool can be damaged. Further, where graphite or a light alloy such as a magnesium alloy is machined, powder-like chips are generated. Because these powder-like chips are scattered over a wide area in the machine tool, a certain time is required to conduct cleaning after the machining, problems of care for workers' health may occur and an adverse effect may be produced on the durability of apparatus.
A tool described in Japanese Translation of PCT Application No. 2005-532917: Patent Document 1 is known as a milling tool that can recover powder-like chips generated when a magnesium alloy is cut. This milling tool has a suction opening formed to be centered on the axis of a shank and the suction opening communicates with a draft hopper mounted on a tightening and feeding device. As the draft hopper is mounted on a tightening and feeding device in the milling tool described in Patent Document 1, such a resultant problem occurs that the structure is complex and the tool cannot be automatically replaced due to the structure.
The applicant has developed a spindle structure disclosed in Japanese Patent Application Laid-open No. 2009-274147 as a chip discharge device of a simple structure that has a spindle structure that enables automatic replacement of tools. In the spindle structure disclosed in Japanese Patent Application Laid-open No. 2009-274147, cutting chips are discharged to the outside from the rear end of the spindle via a through hole formed in the axial center of a draw bar. However, the spindle structure disclosed in Japanese Patent Application Laid-open No. 2009-274147 is not assumed to be a machine tool for machining of works of complex shape by turning a spindle head, as with the universal head disclosed in Japanese Patent Application Laid-Open No. H10-94911: Patent Document 2. Therefore, with the spindle structure disclosed in Japanese Patent Application Laid-open No. 2009-274147, the cutting chips cannot be discharged to the outside from the rear end of the spindle without hindering the turning motion of the spindle head.